1. Field of the Invention
The present invention relates to a method of wet cleaning semiconductor substrates.
2. Description of the Related Art
It Is known in the fabrication of semiconductor devices that the yield of semiconductor devices is greatly affected by the process of wet cleaning semiconductor substrates. In wet cleaning a semiconductor substrate to remove metallic impurities from the substrate surface, it Is important to prevent the removed metallic impurities from adhesion again on the substrate surface and also to prevent traces of metallic impurities contained in the cleaning solution from adhering to the semiconductor substrate surface. If metallic impurities are left on the surface of the semiconductor substrate, the dielectric strength of the silicon oxide film is greatly lowered, and the pn junctions suffer from increased leak currents, resulting in deterioration in the characteristics of fabricated semiconductor devices. As the design rule of recent semiconductor devices becomes finer, metallic impurities remaining on semiconductor substrates have a greater adverse effect on the characteristics and yield of semiconductor devices.
One process of cleaning semiconductor substrates that has been known for a long time is the RCA cleaning process for removing particle impurities and the natural oxide film and metallic impurities from a semiconductor substrate surface. The RCA cleaning process has many modifications, one of which is disclosed in Japanese patent laid-open No. 2-2130 (JP, A, 2-2130). According to the disclosed RCA cleaning process, a semiconductor substrate is first cleaned with an ammonia-hydrogen peroxide cleaning solution composed of ammonia water (29%), hydrogen peroxide water (30%), and water (1 : 1 : 5), and then cleaned In a dilute hydrofluoric acid cleaning solution composed of dilute hydrofluoric acid (50%) and water (3 : 100) to remove a natural oxide film from the semiconductor substrate. Thereafter, the semiconductor substrate is boiled in a hydrochloric acid-hydrogen peroxide cleaning solution composed of hydrochloric acid (36%), hydrogen peroxide water (30%), and water (1 : 1 : 5). In the RCA cleaning process, the ammonia-hydrogen peroxide cleaning solution serves to promote a reaction to dissolve and remove silicon oxide film with ammonia, and also promote a reaction to form a silicon oxide film with hydrogen peroxide, thereby removing fine impurity particles through a combination of these competitive reactions. The dilute hydrofluoric acid cleaning solution dissolves and removes a natural oxide film on the surface of the semiconductor substrate, thus removing metallic impurities from the surface of the semiconductor substrate. When the semiconductor substrate is boiled in the hydrochloric acid-hydrogen peroxide cleaning solution, metallic impurities on the surface of the semiconductor substrate are turned into a soluble chloride which is dissolved and removed, with hydrogen peroxide serving as an oxidizing agent.
The conventional cleaning process described above has the following drawbacks:
While the ammonia-hydrogen peroxide cleaning solution is highly effective in removing fine particles from the substrate surface, since the cleaning solution is alkaline, even if the cleaning solution contains a very low concentration of metallic impurities, e.g., 10.sup.-12 g per 1 g of the cleaning solution, the cleaning solution allows such metallic impurities to adhere to the surface of the semiconductor substrate at a surface density ranging from 1.times.10.sup.12 to 1.times.10.sup.13 atoms/cm.sup.2. Some of the metallic impurities thus deposited on the surface of the semiconductor substrate will not be removed even by the dilute hydrofluoric acid cleaning solution and the hydrochloric acid-hydrogen peroxide cleaning solution. Noble metal elements, typically copper, used as an interconnection material in semiconductor devices are liable to adhere to semiconductor substrate surfaces. When a natural oxide film on a surface of a semiconductor substrate is removed using a dilute hydrofluoric acid cleaning solution, such a noble metal element tends to adhere again to the semiconductor substrate. If the hydrochloric acid-hydrogen peroxide cleaning solution contains metallic impurities such as of iron and aluminum, then they are apt to remain as oxides in the natural oxide film formed on the semiconductor substrate. The hydrochloric acid-hydrogen peroxide cleaning solution is also problematic in that an unnecessary natural oxide film is formed at the same time metallic impurities are removed.
FIGS. 1A, 1B, 1C, and 1D schematically show a process by which a metallic impurity adheres to a semiconductor substrate surface when the dilute hydrofluoric acid cleaning solution is used. When a semiconductor substrate 91 whose surface has already been contaminated by metallic impurities 92 as shown in FIG. 1A is cleaned, the semiconductor substrate 91 is immersed into a cleaning solution 98, and the metallic impurities 92 are separated from the semiconductor substrate 91 into the cleaning solution 98 as shown in FIG. 1B. However, since a natural oxide film has been removed from the surface of the semiconductor substrate 91, some kinds of metallic impurities 92 In the cleaning solution 93 again adhere to the surface of the semiconductor substrate 91 as shown in FIG. 1C. As a consequence, as shown in FIG. 1D, metallic impurities 92 remain on the surface of the semiconductor substrate 91.
FIGS. 2A, 28, 2C, and 2D schematically show a process by which a metallic impurity adheres to a semiconductor substrate surface when a cleaning solution containing metallic impurities is used. As shown in FIG. 2A, only a trace of metallic impurity 92 remains on the surface of a semiconductor substrate 91. When the semiconductor substrate 91 is immersed in a cleaning solution 93 Which contains metallic impurities 92, as shown in FIG. 2B, the metallic impurities 92 are removed from the surface of the semiconductor substrate 91. However, as shown in FIG. 2C, metallic impurities 92 are transferred from the cleaning solution 93 and adhere to the semiconductor substrate 91. As a result, as shown in FIG. 2D, more metallic impurities 92 are present on the cleaned semiconductor substrate 91 than before the semiconductor substrate 91 is cleaned.
In IEICE Trans. Electron, Vol. E75-C, No. 7, pp. 812-818, July 1992, Tsugio Shimono and Mikio Tsuji report a cleaning solution composed of hydrofluoric acid, hydrogen peroxide, and water as a cleaning solution which is suitable for removing metallic impurities from a silicon wafer and which can also be used as an etchant. It is indicated that when a silicon wafer surface is cleaned by the reported cleaning solution, the surface density of metallic impurities on the cleaned silicon wafer is smaller than when cleaned by the conventional cleaning solution described above.